Method of vacuum refining high-temperature alloys

ABSTRACT

Covers a method of making nickel-base alloys characterized by markedly improved rupture ductility particularly in the intermediate temperature range. This includes the steps of charging to a furnace crucible a base charge of carbon and a mixture of nonreactive metals comprising at least nickel, vacuum refining said charge by heating it to a melt at a temperature ranging from about 2,550* F. to about 2,750* F. at a pressure less than 50 microns and completing said refining by further heating the charge to a melt temperature ranging from about 2,800* F. to about 3,000* F. for a period of time until carbon deoxidation is complete. The melt is cooled, at least one reactive metal added thereto, and the nickel alloy is then teemed.

United States Patent Inventors Harold L. Wheaton Kinsington; Edward G. Pekarek, Wllloughby, both of Ohio Appl. No. 876,107 Filed Nov. 12, 1969 Patented Nov. 16, 1971 Assignee TRW Inc.

Cleveland, Ohio METHOD OF VACUUM REFINING HIGH- [56) References Cited UNITED STATES PATENTS 3,304,176 2/l967 Wlodek 75/171 Primary Examiner- Richard 0. Dean Attorney-Hill, Sherman, Meroni, Gross & Simpson ABSTRACT: Covers a method of making nickel-base alloys characterized by markedly improved rupture ductility particularly in the intermediate temperature range. This includes the steps of charging to a furnace crucible a base charge of carbon and a mixture of nonreactive metals comprising at least nickel, vacuum refining said charge by heating it to a melt at a temperature ranging from about 2,550 F. to about 2,750 F. ata pressure less than 50 microns and completing said refining by further heating the charge to a melt temperature ranging from about 2,800" F. to about 3,000 F. for a period of time until carbon deoxidation is complete. The melt is cooled, at least one reactive metal added thereto, and the nickel alloy is then teemed.

METHOD or VACUUMREFINING HIGH- rrsmrsrmruna ALLOY Field of the Invention The invention relates to a method of vacuum refining hightemperature nickel-base alloys whereby the resultant nickelbase alloy exhibits greatly increased rupture ductility particularly in the intermediate temperature range of about l200-l 500 F. The unique high-temperature properties of these alloys are achieved through the addition of a variety of alloying elements. Chromium is added to impart oxidation and hot corrosion resistance and the quantity used is dependent on the level of resistance required. Generally about 3 to 20 percent is present. One or more metals from the group consisting of tungsten, molybdenum, columbium, tantalum and vanadium are used for solid solution strengthening. Alloys may contain up to 25 percent tungsten, up to 12 percent molybdenum, up to 12 percent tantalum, up to 6 percent columbium, and up to percent vanadium. Aluminum and titanium are added to form a finely dispersed precipitate which imparts high temperature strength at quantities of 2 to 8 percent aluminum and 2 to 6 percent titanium may be present. Up to 0.5 percent carbon, 0.2 percent boron, 0.5 percent zirconium and 3 percent hafnium may also be present to improve the high-temperature strength and ductility of the alloy.

Description of the Prior Art High-strength cast nickel-base alloys of the type previously described are used in a wide variety of high-temperature structural applications such as turbine blades and the like. Alloys of this type are generally made by vacuum refining a nonreactive metal base charge in the presence of carbon to deoxidize the melt. The charge is usually heated under a condition of vacuum refining constituting the carbon reduction cycle at a temperature ranging from about 2,600 to about 2700 F. The pressure in the vacuum system should be less than 100 microns and preferably less than 50 microns. The vacuum refining is carried out until a constant leak-up rate is achieved which indicates completion of refining.

Once this step is complete, the reactive elements Al, in the alloy are added to the melt. The reactive elements are those which have a higher affinity for oxygen than carbon has and included in this group are Al, Ti, Zr, B and Hf. Heats prepared in this manner are generally cast into ingots which are subsequently remelted in smaller vacuum furnaces and cast into shell or investment molds.

However, conventional high-strength cast nickel-base alloys produced by the above conventional vacuum refining techniques have been found to suffer from low rupture ductility, particularly in the intermediate temperature range, around l200-l 500 F. Because of their low ductility chances for premature failure materially increase.

lt would therefore be a considerable advance in the art if a method of providing cast nickel-base alloys of high strength were discovered, which alloys particularly exhibited improved ductility in the intermediate temperature range. It would be particularly advantageous if this property of ductility were increased without sacrifice of other valuable alloy properties. Further, it would be a great benefit to the art if such process could achieve the above in a simple, yet efficient manner utilizing existing equipment, and without recourse to addi tional time-consuming steps as compared to the now conventional technique.

SUMMARY OF THE INVENTION The present invention relates to a method of producing greatly improved high-strength cast nickel-base alloys, which are particularly characterized as exhibiting relatively high ductility in the intermediate temperature range. Broadly speaking, the invention includes the steps of first charging to a furnace crucible a base charge containing, in addition to carbon', a mixture of nonreactive metals comprising at least nickel. The charge is vacuum refined at a pressure less than microns by heatingiit to'a melt temperature ranging from about '2550*F. to about'2750 F. untilzthe carbon boil subsidesfThis time-willvary depending onthe gas content of the base charge? The molten charge .isthen further heated at a temperature ranging from about 2800 F. to about 3000 F. for a period of time until carbon deoxidation is considered complete as indicated by the attainment of a constant leak-up rate. Thereafter, the melt is cooled, at least one reactive metal added to the melt, and the resultant nickel alloy melt is teemed.

It therefore becomes the object of the invention to provide a method of making an improved nickel alloy.

A more specific object of the invention is to provide a process of making a nickel alloy, which is particularly characterized as exhibiting high ductility in the intermediate temperature range.

Yet another object of the invention is to provide a method of making the above nickel alloy which while possessing high ductility also possesses the other necessary and desirable properties of conventionally cast nickel alloys, such as high strength, etc.

Another object of the invention is to provide a method of making said improved nickel alloy utilizing only conventional equipment without resort to costly and time-consuming additional multiple steps, as compared to prior art refining techniques.

A still further object of the invention is to provide a method of making said nickel alloy, which process is applicable to alloying a wide variety of metals and metalloids in conjunction with nickel.

Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure.

DESCRlPT ION OF THE PREFERRED EMBODIMENTS The first step in the invention is to provide a base charge of nonreactive metals. In addition to nickel the charge may also include a wide variety of additional metals such as cobalt, chromium, molybdenum, tantalum, tungsten, columbium, vanadium and mixtures thereof. The metals are charged. to a furnace crucible along with a carbon source used to ac'complish deoxidation. A preferred carbon source is graphite, most preferably in the powdered form.

After the furnace is charged, it is then placed under a vacuum. Pressures on the order of less than 100 microns, preferably not in excess of about 50 and most preferably less than 10 microns of mercury are appropriate.

The metals are then heated to a temperature ranging from about 2550 F. to about 2750 F. Generally, they are held at this melt temperature from about one-sixth hour to about 5 hours. This allows the initial boil to subside so that melt can be heated to higher temperature without danger of the boil becoming too vigorous.

Thereafter, the temperature is increased and the vacuum refining step continued at a temperature ranging from about 2800 F. to about 3000 F. Again, the melt is held at this temperature for a period of time ranging from about one-sixth hour to about 5 hours. Generally, refining is considered complete when the rate of gas evolution reaches an equilibrium or becomes constant as determined by measuring leak-up rates. Leak-up rates are determined by isolating the melting chamber of the vacuum furnace from the pumping system by closing the valve which separates these areas. After a given time interval (e.g. 30 seconds), the pressure rise is determined by subtracting the pressure at the time the valve was closed from the pressure after the time interval. Leak-up rates are usually expressed in microns per 30 seconds.

The melt is then cooled down to a temperature ranging from about 2600 F. to about 2700 F. prior to addition of reactive metals. The entire time of heating in the two-stage heating process should take at least 3 hours. This includes the time to heat up to the initial lower temperature and higher temperature, duration of heating at both temperature levels, and time to cool down to the initial temperature range just prior to addition of reactive metals.

Again, a wide variety of known reactive metals may be added at this stage of the process including metals such as aluminum, zirconium, nickel-zirconium, nickel-boron, titanium,

etc.

After a period of time necessary to insure homogeneity of the melt, say about one-half hour, the resultant nickel alloy melt is then teemed by pouring it into molds. Homogeneity and metal cleanliness are sometimes aided by superheating the melt after the reactive additions, i.e. raising to temperatures from 260072700" F. to 2800/3000' F.

The key step of the invention, of course. lies in the second high-temperature heat step. l-leretofore, it was thought that refining at such a high temperature would increase refractory contamination due to increased attack of the refractory crucible by the metal. However. such does not occur here as was expected and the highdemperature heating step surprisingly imparts increased intermediate temperature ductility to the alloy, compared to the same alloys produced by the conventional vacuum refining technique involving only a low temperature (2550'-2750' F.) step. Moreover, no contamination of the alloys produced in accordance with the invention is noted. It is believed that during the heating cycle a skull is formed which may offer partial protection to the refractory crucible from attack by the melt.

The temperature range in the secondheating step is particularly critical if maximum ductility is to be achieved. Below Example I An alloy consisting of 0.10 carbon, 8.0 chromium. l0.0

. 700 F. was reached. The melt was held at this temperature for 2800' F. complete refining is not obtained, while about 3000' F. contamination of the melt by the refractory crucible may occur. Thus, it is important that the temperature be maintained within the 2800'-3000 F. range in the second heating step.

' The method of the invention described is applicable to refining of a wide variety of nickel alloys. One series of alloys which may be processed by the steps outlined above contain the following components expressed in percentages by weight:

0.05-0.l5 carbon 7-% chromium 7-l ll cobalt 4-85: tantalum 3-7% molybdenum moi-0.02% boron 0.05-010% zirconium Balance-Essentially nickel.

0.7$-l.25l titanium 5.5-6.5 aluminum 0-35 hafnium Iii-2.5% titanium (fl-5.5% aluminum 010-0201: carbon l-IO'I: chromium 9-l l5 cobalt 0.0l-0.02% boron 0.54.51 columbium ODS-0.08% zirconium I l-Hi tungsten 04% hafnium Balance-essentially nickel.

A still further series of alloys usefully refined here contains the following components in terms of percentages by weight:

0.04-0 I5! carbon 4-55 aluminum ls-Idi chromium "-16% cobalt I 3-51 molybdenum 0.0054103'11 boron 04% hafnium 34% titanium Balance-essentially nickel.

about one-half hour. After the melting had begun, additional nickel, chromium, cobalt and carbon were added as well as molybdenum and nickel-tantalum. After the rate of gas evolu-' tion had been detennined to be constant at the above temperature range the temperature was increased to 2800'-3000 F. The melt was held at this temperature for about 1 hour at which time the rate of gas evolution again reached an equilibrium. g r

The temperature of the melt was then reduced to 2600'-2 700' F. and reactive materials added. Aluminum, zirconium, nickel-boron and titanium were added at this time. Prior to charging of the reactive elements the molten bath had been allowed to cool without being stirred. This practice aided in the cleanliness of the melt since any small slag" particles that might be present are given time to float to the top of the bath. When heat is again applied, these particles coalesced as the metal was stirred inductively and were driven to the crucible wall where they adhered.

Thereafter, the melt was then poured into tube mold clusters. The tube molds had been grit-blasted and coated on the inside diameter with acetylene soot. The alloy was poured under vacuum and so maintained for 24 hours before venting to the atmosphere.

An alloy produced in accordance with the above method was then tested for ductility. This particular alloy had a 2.7 percent creep ductility at M00 F. and under an 85,000 p.s.i.

stress and had a rupture life of a about 250 hours. An exact similar alloy was produced without resort to the high-temperaturc step, and was vacuum refined solely within the range of 2600-2700 F. This alloy, when tested at i400" F. under a stress of 85,000 p.s.i., had a l.4 percent creep ductility and a 50-hour rupture life. Thus, it is quite evident that the second heating step at a relatively high temperature was extremely beneficial in imparting to a nickel alloy greatly improved ductility, especially within an intermediate temperature range.

Example ll temperature step and was vacuum refined solely within the range of 2500-2700 F. This alloy, when tested at l800 F.

under a stress of 29,000 p.s.i., had a rupture life of 65 hours,

an elongation of 5.8 percent and a reduction of area of 6.5 percent.

7 Example Ill A number of additional runs were made refining either the alloy example i or example ll. The various times involved in carrying out the heating steps of the invention are summarized in tables land [I with respect to these runs.

TABLE l End refining at Melt at End refining Alter cool to All Melted ,S50*2,900" F 2,8502,900 F. 2,850--2,900 F. 2,6002,700 F. R

Time Pr. R 2 Time lr Rot- Time Pr. Roe Time lr R01 'limu Pr R00 No 4:20 12 10 4:42 3 8 :02 12 14 0:03 0 7:40 3 0 1 4:25 7 10 4:45 0 10 5:30 20 30 7:45 10 20 0:10 5 7 2 3:14 10 18 4:53 10 10 5:15 22 23 7. 10 22 0:30 4 10 3 5:43 3 0:01 7 7 0:32 5 5 0:50 5 5 3:57 10 4 4 3:50 7 12 4:07 7 10 4:30 0 11 5:33 0 10 0:50 0 0 5 3:40 7 7 3:57 0 7 4:40 5 a 5:01 5 8 0:53 4 4 0 4:40 12 0 4:53,. 12 3 5:25 13 7:45 13 10 0:11 7 7 7 4:42 5 5 4:53 5 5 5:20 0 3 5:47 3 it 0:50 4 4 3 3:15 0 4 3:2 5 3:40 7 0 4:10 0 3 0:15 4 4 0 5:05 3 5:15 3 5 5:43 0 14 0:00 4 10 3:05 2 3 10 4:31 10 10 4:47 10 10 5:23 18 13 0:42 3 10 7:44 3 0 11 4:32 7 4:47 0 7 5:15 3 12 0:12 0 10 7:32 4 4 12 4:15 5 4:20 5 0 5:00 7 10 5:21 5 10 7:12 3 4 13 4:00 8 4:14 7 10 4:35 24 22 0:00 0 10 7:43 3 5 14 12:48 3 13:24 24 3 14:03 2 4 14:22 15 4 15:31 2 3 15 6:25 2 0:40 2 3 7:20 2 3 7:40 2 3 0:25 2 10 5:45 3 0:03 3 3 0:44 2 7:05 2 0 3:00 2 7 17 8:15 3 8:38 2 0 0:30 2 0 0 2 0 11.00 1 3 13 8:20 0 8:41 5 0 0:14 4 1 2 4 0 11:00 4 0 10 7:40 4 8:06 4 3 3:20 2 3:37 2 5 10:40 2 20 1 Pr.=pres5ur0'microns. 1 ROE =rate of gas ivolutio11n1icrons 30 seconds.

TABLE II Total time Time to Total time Time to cool to of low bring 2,600" F. Total time temperto high 2,700 F. molten heat ature. temper- W refining. ature, Min- Minminutt's minutes Hours ntcs Hours 1110 1 22 1 1 37 3 2 30 15 1 31 4 5 30 50 1 10 3 1 1s 24 2 1 3 1 17 3 1 17 3 11 21 1 52 3 7 13 20 1 20 4 31 11 27 1 0 2 4 11 2 2 10 3 5 10 20 1 50 3 10 14 1 2 3 13 15 57 1 20 3 11 21 1 51 2 57 14 1 43 3 43 30 10 1 0 2 43 21 11 1 45 3 23 21 55 2 15 23 10 1 20 2 45 21 8 1 33 2 40 20 x 2 3 3 A number of samples from the above runs were also tested mediate temperature range which comprises the steps of for creep rupture and compared to a test sample derived from h rging t0 3 f rna Cr ible a ba e charge of carbon and he conventional process, that is, involving only the low-heat nonreactive metals comprising at least nickel, vacuum refining step: This latter example is designated as Run A. As is readily said charge by heating it to a melt at a temperature ranging evident from table lll below, all the samples prepared accordr about 2550 I abOI-It 2750 further a g Said ing to the process of the invention had materially g ate charge at a melt temperature ranging from about 2800" F. to creep rupture properties in terms of enhanced life hours and about 3000 F. for a period of time until carbon deoxidation is percent creep compared to the sample produced by th oo complete, cooling said melt, adding at least one reactive metal ventional prior art process. to said melt and teeming said nickel alloy melt.

2. The method of claim 1 wherein said melt is cooled to TABLE about 2500-2700 F. prior to addition of reactive metal.

3. The method of claim 1 wherein the total time of heating L400. F390 Ksi prior to addition of reactive metal is at least about 3 hours. Run No Life Hrs. Creep 4. The method of claim I wherein both the initial heat step and final heat step are carried out at a time ranging from about one-sixth hour to about 5 hours. A 53.2 (85 Km.) 1.3 I H10 33 5. The method of claim I wherein said base charge also con- 2 71.3 2.9 tains at least one metal selected from the group consisting of Q 3 25.; 2.8 cobalt, chromium, tantalum, molybdenum, tungsten, columbi- 5 um, vanadium, and mixtures thereof. 6 69,5 6. The method of claim 1 wherein said melt is heated to 7 02.5 4.5 about 2800 to about 3000 F. after the addition of the reac 3:: tive metals. 7. The method of claim I wherein said reactive metal is II 5119 2.3 selected from the group consisting of aluminum, zirconium, 613 boron, titanium, hafnium and mixtures thereof 8 The method of claim I wherein the carbon added is V I present in the form of graphite powder. We claim asourmvenuon: 9. The method of claim I wherein said carbon deoxidation l A method of making an improved nickel alloy particuis considered complete when the rate of gas evolution essenlarly characterized as exhibiting high ductility in the intertially reaches equilibrium 10. The method of claim 1 wherein the alloy is comprised of, by weight, 0.05 to 0.15 percent carbon, 7 to 9 percent chormium, 9 to 11 percent cobalt, 4 to 8 percent tantalum, 3 to 7 percent molybdenum, 0.75 to 1.25 percent titanium, 5.5 to 6.5 percent aluminum, 0.01 to 0.02 percent boron, 0.05 to 0.10 percent zirconium, up to 3 percent hafnium with the balance essentially nickel.

11. The method of claim I wherein the alloy is comprised of, by weight, 0.10 to 0.20 percent carbon, 8 to 10 percent chromium, 9 to l 1 percent cobalt, 0.5 to 1.5 percent columbium, 11 to 14 percent tungsten, 1.5 to 2.5 titanium, 4.5 to 5.5 aluminum, 0.01 to 0.02 boron, 0.03 to 0.08 zirconium, up to 3 percent hafnium, with the balance essentially nickel.

12. The method of claim 1 wherein the alloy is comprised of, by weight, 0.04 to 15 percent carbon, 14 to 16 percent chromium, 3 to 5 percent molybdenum, 3 to 4 percent titanium, 4 to 5 percent aluminum. 14 to 16 percent cobalt, 0.005 to 0.03 boron, up to 3 percent hafnium, with the balance essentially nickel.

* n aaa Patent No. 9 Dated November 16, 1971 lnventofls) Harold L. Wheaton 81 Edward G. Pekarek It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 24, "2 to 6 percent" should be 2 to 6%-;

Column 1, line 41, "elements Al, in" should be elements used Column 4, line 62, "2500-2700F. should be 2600-2700 F.

Column 5, Table III,

ll 58. 9 2. 3" should be Signed and sealed this 8th day of August 1972.

(SEAL) A ttest:

EDWARD ILFLETCHER JR ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents iM PO-IOSO (10-697 USCOMM-DC 50376-P69 U 5. GOVERNMENT PRINTING OFFICE I969 OJ6$-J3l Inventor's address, listed as "Kinsington" should be --Kensington--; 

2. The method of claim 1 wherein said melt is cooled to about 2600* -2700* F. prior to addition of reactive metal.
 3. The method of claim 1 wherein the total time of heating prior to addition of reactive metal is at least about 3 hours.
 4. The method of claim 1 wherein both the initial heat step and final heat step are carried out at a time ranging from about one-sixth hour to about 5 hours.
 5. The method of claim 1 wherein said basE charge also contains at least one metal selected from the group consisting of cobalt, chromium, tantalum, molybdenum, tungsten, columbium, vanadium, and mixtures thereof.
 6. The method of claim 1 wherein said melt is heated to about 2800* to about 3000* F. after the addition of the reactive metals.
 7. The method of claim 1 wherein said reactive metal is selected from the group consisting of aluminum, zirconium, boron, titanium, hafnium and mixtures thereof.
 8. The method of claim 1 wherein the carbon added is present in the form of graphite powder.
 9. The method of claim 1 wherein said carbon deoxidation is considered complete when the rate of gas evolution essentially reaches equilibrium.
 10. The method of claim 1 wherein the alloy is comprised of, by weight, 0.05 to 0.15 percent carbon, 7 to 9 percent chormium, 9 to 11 percent cobalt, 4 to 8 percent tantalum, 3 to 7 percent molybdenum, 0.75 to 1.25 percent titanium, 5.5 to 6.5 percent aluminum, 0.01 to 0.02 percent boron, 0.05 to 0.10 percent zirconium, up to 3 percent hafnium with the balance essentially nickel.
 11. The method of claim 1 wherein the alloy is comprised of, by weight, 0.10 to 0.20 percent carbon, 8 to 10 percent chromium, 9 to 11 percent cobalt, 0.5 to 1.5 percent columbium, 11 to 14 percent tungsten, 1.5 to 2.5 titanium, 4.5 to 5.5 aluminum, 0.01 to 0.02 boron, 0.03 to 0.08 zirconium, up to 3 percent hafnium, with the balance essentially nickel.
 12. The method of claim 1 wherein the alloy is comprised of, by weight, 0.04 to 15 percent carbon, 14 to 16 percent chromium, 3 to 5 percent molybdenum, 3 to 4 percent titanium, 4 to 5 percent aluminum, 14 to 16 percent cobalt, 0.005 to 0.03 boron, up to 3 percent hafnium, with the balance essentially nickel. 